Liquid gauge having an optical fiber with flexible membrane enclosure

Abstract

A fiber optic fuel or liquid level gauge is disclosed which determines the liquid level by measuring the amount of light loss due to evanescent transfer to the fuel or liquid. An optical fiber is supported in the fuel or liquid tank, with a first end adjacent the highest possible liquid level to be measured with a reflector at the first end. The optical fiber extends downwardly through the tank, and its second end is connected to a light source for injecting light into the fiber. The intensity of the input light is compared to the intensity of light reflected from the first fiber end, and the liquid level is calculated from the light loss. The gauge has very high reliability since it has no moving mechanical parts.

what is claimed is: 
     
       1. A fiber optic liquid level gauge for measuring the level of a first liquid in a container, comprising: an optical fiber disposed within the container and extending through the range of liquid surface positions to be measured by the gauge, wherein the optical fiber is characterized by an inner fiber core and an outer fiber cladding, the thickness of fiber cladding on the fiber portion which extends through said range of positions being selected to provide significant evanescent wave loss when the cladding is immersed in the liquid;   a light source for injecting light into said fiber;   measuring means for measuring the degree of light intensity loss at the fiber/liquid interface, and providing an intensity loss signal indicative of the light intensity loss;   liquid level indicating means responsive to said intensity loss signal for indicating the liquid level in the container; and   a quantity of a second liquid disposed within the flexible membrane, whereby the surface level of the second liquid within the flexible membrane is indicative of the surface level of the first liquid within the tank.   
     
     
       2. The gauge of claim 1 wherein the second liquid is characterized by a surface tension such that the second liquid does not wet the optical fiber. 
     
     
       3. A fiber optic liquid level gauge for measuring the level of a first liquid in a container, comprising: an optical fiber having a first end and a second end disposed in said container and extending within the container through the range of liquid surface levels to measured, the optical fiber characterized by an inner fiber core and an outer fiber cladding, the thickness of the fiber cladding being selected to provide significant evanescent wave loss at fiber/liquid interfaces;   reflector means at said first end of said optical fiber;   a light source for injecting light into said second end of said fiber;   a first optical coupler for coupling off a portion of the light being injected into the optical fiber;   a second optical coupler adjacent said second end for coupling off a portion of the light which has traversed the fiber and been reflected from the optical reflector at the first end;   means for comparing the light intensity from the first optical coupler with the light intensity from the second optical coupler to determine the liquid level; and   a quantity of a second liquid disposed within the flexible membrane, whereby the surface level of the second liquid within the flexible membrane is indicative of the surface level of the first liquid within the tank.   
     
     
       4. The gauge of claim 3 wherein the second liquid is characterized by a surface tension such that the second liquid does not wet the optical fiber.